We intend to quantify, characterize and compare the spectrum of mutations produced by ionizing radiations in human x hamster hybrid AL cells and in mouse lymphoblastoid L5178Y tk+/- cells using quantitation of loss of phenotype and molecular and cytogenetic analyses. We will focus on multilocus, chromosomal mutations. Later we intend to extend these analyses to a new hybrid construct (XR1/11) which is deficient in repair of double strand breaks but which, like AL, contains human chromosome 11. We also propose to characterize mutation in a line of human fibroblasts (EJ30) made heterozous at the thymidine kinase (tk) and/or S1 antigen locus. Our goals are to determine: (1) if different kinds, or different proportions, of particular mutations are produced at different loci; (2) how human and rodent cell lines may differ in this regard; (3) if ionizing radiations produce a particular and specific spectrum of mutations; and (4) the role of repair in mutagenesis. Mutations induced by X-ray, gamma rays and neutrons will be quantified by selection for the homogzyous tk-/- phenotype in mouse L5178Y and human EJ30 cells and for loss from AL and XR1/11 cells of the surface antigens (S1,S2,S3) coded by the human chromosome 11. Mutation in hypoxanthine guanine phosphoribosyltransferase (hprt) will provide a quantitative benchmark; UV and/or ethylmethane sulfonate (EMS) will serve as comparison mutagens. We will seek signature mutations unique to radiation characterized by DNA probes and other recombinant techniques.